It is necessary to understand the role of the final motor pathway, the motor units, to comprehend how the nervous system controls limb movement. It has recently been shown that (1) cat hindlimb muscles contain three motor unit types; and (2) each motor unit has an "optimal" frequency of firing which maximizes the momentum output (force-time integral) from the muscle for any given number of unit discharges greater than two. This proposed research is to study the role of single motor units and of different motor unit types during locomotion in the mesencephalic cat. Specifically, discharge patterns of individual units during locomotion will be analyzed to determine (1) whether or not motor units discharge at rates equal to their "optimal" frequency for maximizing momentum output, an important physical quantity in the locomotion of animals; (2) the participation of each motor unit type; (3) the fatigue properties of motor units; and (4) the recruitment order of different size motor units to test the "size principle" hypothesis. The discharge patterns of motoneurons corresponding to single motor units will be recorded with micropipettes inserted into the spinal cord of the mesencephalic cat during treadmill locomotion controlled by brainstem stimulation. Afterwards, the motoneurons will be stimulated and muscle tension recorded to determine momentum output and motor unit types.